Combination of O2 and CO2-derived variables to detect tissue hypoxia in the critically ill patient

Oxygen-derived parameters have been traditionally used to guide resuscitation during shock states. Nevertheless, normalization of venous oxygen saturation does not exclude the persistence of tissue hypoperfusion and tissue hypoxia. Combination of O 2 and CO 2 -derived variables has consistently demonstrated to be related with clinical outcomes and its variations could anticipate changes in lactate and also predict fluid responsiveness in terms of oxygen consumption. Here we discuss the potential mechanisms leading to increase the venous-to-arterial CO 2 (Cv-aCO 2 ) to arterial-to-venous O 2 content difference (Ca-vO 2 ), i.e., the Cv-aCO 2 /Ca-vO 2 ratio, its potential clinical application, limitations and uncertainties. Finally, although biologically plausible, the potential applications of the Cv-aCO 2 /Ca-vO 2 ratio in the clinical practice require to be confirmed.